Not applicable.
1. Field of Invention
This invention relates to fiber-coupled laser diodes, specifically to fiber-coupled laser diodes that have both high coupling-efficiency and low feedback-noise.
2. Description of Prior Art
A fiber-coupled laser diode uses a fiber to transmit the laser diode light. Since the light emitted from the laser diode has already been launched into a fiber, a fiber-coupled laser diode can be readily used in optical-fiber networks for communications and sensing. In addition, a fiber-coupled laser diode is a flexible coherent light source, which can also be used in many optical devices, such as biomedical equipment, optical disk systems, laser printers, and others. A good fiber-coupled laser diode must provide both high coupling-efficiency to maximize the optical power output from the fiber and low feedback-noise to eliminate frequency and intensity fluctuations in the output light.
In order to obtain high coupling-efficiency between a laser diode and a single-mode fiber, i.e., to maximize the transmitted optical power through the fiber, the laser diode light distribution must match the node of the fiber. (See for example, M. Saruwatari and K. Nawata, xe2x80x9csemiconductor laser to single-mode fiber coupler,xe2x80x9d Applied Optics, Vol. 18, 1847-1856 (1979); M. Cote and R. R. Shannon, xe2x80x9cOptimization of waveguide coupling lenses with optical design software,xe2x80x9d Applied Optics, Vol. 35, 6179-6185 (1996)) The mode of a single-mode fiber has a circular Gaussian distribution. If the light distribution does not match the mode of the fiber, only part of the light enters and propagates through the fiber. Consequently, the output optical power from the fiber is low. On the other hand, if the light distribution matches the mode of fiber, the entire light will enter and propagate through the fiber. Thus, the coupling efficiency is nearly 100%. In other words, almost all optical power emitted from the laser diode is transmitted through the fiber.
The light emitted from a laser diode has an elliptical Gaussian distribution and astigmatic aberration. Thus, the emitted light must be corrected to a circular Gaussian beam, which must also be free from astigmatic aberration. A method for correcting the laser diode light using a microlens is described in J. J. Snyder, xe2x80x9cCylindrical micro-optics,xe2x80x9d Proceedings of SPIE, Vol. 1992, 235-246 (1993); S. Jutamulia, xe2x80x9cCorrection of laser diode beam using microlens optics,xe2x80x9d Optical Memory and Neural Networks, Vol. 10, 113-116 (2001); and U.S. Pat. No. 5,181,224 to Snyder (1993).
After the laser diode light is corrected to be free from astigmatic aberration, and to have a circular Gaussian distribution, the laser diode light is focused using an imaging lens onto the entrance end of the single-mode fiber. The beam-waist of the corrected Gaussian beam can be varied to match the mode of fiber by adjusting the distance from the microlens to the imaging lens, and the distance from the imaging lens to the fiber.
A prior-art fiber-coupled laser diode using a microlens is schematically shown in FIG. 1. The astigmatic and elliptical beam emitted by a laser diode 20 is corrected by a microlens 22 to an astigmatic-aberration-free, circular Gaussian beam. The corrected beam is then focused by an imaging lens 24 to enter a single-mode fiber 26.
Although high coupling-efficiency can be expected since the light distribution matches the mode of fiber, it has a severe drawback, i.e., high feedback-noise, as will be discussed in the following section. The feedback noise will generate frequency and intensity fluctuations in the output light. Thus, it will seriously limit the usefulness of the fiber-coupled laser diode.
A laser diode is a light-emitting device based on a light amplification effect. Light is amplified when it passes inside a laser diode. Similar to other lasers, a laser diode has a resonator. The resonator has a frequency response, while the light amplification effect has another frequency response. The overlapping area of two frequency responses will determine the lasing frequency (i.e., the frequency of the laser light). If no light from outside the resonator enters the laser diode, the light emitted by the laser diode will have a stable intensity and frequency. However, if light from outside the resonator enters the laser diode, the outside light will also be amplified and interfere with the light generated inside the resonator, resulting in frequency and intensity fluctuations in the emitted light. The outside light includes the light emitted from the laser diode and then reflected back into the laser diode. The reflection of a fraction larger than 10xe2x88x926 of the emitted optical power is sufficient to generate the intensity and frequency fluctuations. (W. Bludau and R. H. Rossberg, xe2x80x9cLow-loss laser-to-fiber coupling with negligible optical feedback,xe2x80x9d Journal of Lightwave Technology, Vol. LT-3, 294-302 (1985))
The prior-art fiber-coupled laser diode shown in FIG. 1 suffers from severe intensity and frequency fluctuations caused by two feedback-noise sources: (1) reflection from microlens 22, because it is very close (about 30 xcexcM) to laser diode 20, and (2) reflection from the end of fiber 26, because the light reflected from the end of fiber 26 is focused back by lens 24 and microlens 22 to laser diode 20.
Antireflection (AR) coatings reduce the reflection to only about 10xe2x88x923 of the incoming power which is not sufficient to suppress the feedback-noise. To reduce the reflection from the end of fiber 26 to enter the laser diode, an angled fiber can be used (i.e., the end of fiber is polished to form an angle that is not perpendicular to the fiber axis). However, this will substantially reduce the coupling-efficiency, since the incoming light will be bent at the entrance end of angled fiber when it enters the angled fiber. The mode matching condition is no longer preserved when the incoming light is bent. The reflection from microlens 22 alone, which is AR coated, is sufficient to generate intensity and frequency fluctuations These unwanted fluctuations limit the application of the apparatus. For example, the intensity fluctuation precludes its use in optical disk systems and the frequency fluctuation precludes its use in laser diode pumped solid-state lasers.
An attempt to make a fiber-coupled laser diode having both high coupling efficiency and low feedback-noise has been made using a microlens, which is directly fused to the fiber. (W. Bludau and R. H. Rossberg, xe2x80x9cLow-loss laser-to-fiber coupling with negligible optical feedback,xe2x80x9d Journal of Lightwave Technology, Vol. LT-3, 294-302 (1985)) Although the feedback-noise can be suppressed to 10xe2x88x927 of the emitted light power by placing the microlens exceeding 100 xcexcm away from the laser diode and directly fusing the microlens to the fiber, it fails to obtain high coupling-efficiency. The coupling efficiency is only between 40% and 70%. The fused microlens is fabricated based on an empirical method, instead of precise design and production. Thus, the microlens suffers from severe aberrations, and the coupling efficiency cannot be high. In addition, it cannot be produced in volume.
To summarize, the prior-art fiber-coupled laser diode shown in FIG. 1 is capable of providing a coupling efficiency as high as  greater than 90%. (S. Jutamulia, xe2x80x9cOptical communications: technology and economy,xe2x80x9d Proceeding of the 2002 International Conference on Opto-Electronics and Laser Applications, A-07-A-10 (2002) ISBN: 979-8575-03-2) However, it suffers from severe intensity and frequency fluctuations generated by feedback noise. On the other hand, a prior-art fiber-coupled laser diode using a microlens filed to the fiber may suppress feedback noise, but its coupling efficiency is low ( less than 70%).
Accordingly, several objects and advantages of the present invention are:
(1) to provide an improved fiber-coupled laser diode;
(2) to provide a fiber-coupled laser diode with high laser-to-fiber coupling efficiency;
(3) to eliminate intensity and frequency fluctuations in the output light of a fiber-coupled laser diode;
(4) to provide an astigmatic-aberration-free, circular collimated-laser-diode-beam; and
(5) to eliminate intensity and frequency fluctuations in the output astigmatic-aberration-free, circular collimated-laser-diode-beam.
Further objects and advantages are to provide a fiber-coupled laser diode, which is simple to use and maintain and which is inexpensive to manufacture in volume. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.
In accordance with the invention, a laser diode is provided with a collimating lens, together with a pair of cylindrical lenses or a cylindrical lens having both positive (convex) surfaces to correct the astigmatic and elliptical laser diode light to an astigmatic-aberration-free, circular collimated-beam. The corrected beam can provide high coupling-efficiency between the laser diode and a single-mode fiber. The beam is then input to a low-feedback-noise fiber collimator, resulting in a fiber-coupled laser diode having high coupling-efficiency and low feedback-noise.